I. Field of the Invention
The present invention relates generally to the preservation of wood containing objects and, in particular, to wood containing objects such as museum artifacts, and musical instruments by controlling the humidity of the storage environment for such items.
II. Related Art
Wood products stored in enclosures will absorb and release moisture as dictated by the temperature and storage environment humidity. Unfortunately, as moisture content changes, wood will shrink and expand in a manner that will often change the physical properties and appearance permanently and may damage a stored wood product. It is well known, for example, that in cold weather, particularly in polar latitudes, the indoor moisture content is often very low and low humidity levels cause wood products to shrink in a manner that may cause damage. Conversely, in high humidity environments such as tropical regions, the high humidity may well damage wood products due to moisture absorption and swelling. Because of concerns with environmental humidity, highly valued wood products such as art objects, musical instruments, and even expensive sports equipment are frequently sealed in a controlled humidity environment. The recommended relative humidity (RH) for the storage of wood products has been found to be between 40% and 60%.
As disclosed in U.S. Pat. No. 5,936,178, the relative humidity of closed environments can be stabilized by the use of humidity control systems comprised of moisture permeable pouches containing specific salt solutions. Also, certain non-ionic solution containing low molecular weight molecules such as ethylene glycol, propylene glycol, glycerin, urea, guanidine, ethanol amine, simple sugars, or sugar alcohols may be employed. The choice of the solute and the solute concentration in an aqueous solution determines the solution water activity (aw). The water activity of a solution will equilibrate with the surrounding atmosphere until the relative humidity (RH) is equivalent to the water activity. Thus, the RH percentage will equal the aw×102. For example, an aqueous solution of saturated sodium chloride with excess crystals, aw 0.75 will equilibrate with the moist air or dry air in a closed system until a relative humidity of 75 percent is achieved.
However, it has also been discovered that many of the solutions that are most effective in establishing and maintaining the optimal relative humidity range (40% to 60%) have undesirable properties such as chemical reactivity with packaging material, corrosiveness, undesirable gas formation, or limited moisture transfer capacity (MTC) (defined below). For example, a saturated solution of potassium carbonate with excess crystals will maintain a stable humidity in the desired 40% to 60% range, but the high pH of the material will degrade many packaging films. Another problem is the corrosive gas that is formed when nitrite salts are used to control the humidity.
The moisture transfer capacity (MTC) is defined as the amount of moisture transferred, into or out of a given control system, i.e. device, pouch, etc., over a defined relative humidity range. For example, an aqueous solution of calcium chloride, magnesium chloride, or glycerin may be used to control humidity between 40% and 60%, but the MTC is only about 15 to 20% of the package weight. Also, moisture control systems employing solid substrates such as silica gel or water absorbing organic polymers, e.g. acrylamide, can be formulated to a specific relative humidity; however, these systems are characterized by low moisture transfer capacity and are less useful for maintaining a controlled humidity environment. Clearly the moisture transfer capacity of a humidity control system is very important for maintaining the desired humidity over a range of environmental conditions. As the moisture transfer capacity of the device increases, there is an increased ability of the device to provide the necessary moisture to maintain the humidity required to protect a wood product of interest.